Embodiments of the present invention generally relate to spinal cord stimulation and more particularly to feedback control of stimulation as a lead moves with respect to the dorsal column of the spinal cord.
Neurostimulation systems are devices that generate electrical pulses and deliver the pulses to nerve tissue to treat a variety of disorders. Spinal cord stimulation (SCS) is the most common type of neurostimulation. In SCS, electrical pulses are delivered to nerve tissue in the spine typically for the purpose of chronic pain control. While a precise understanding of the interaction between the applied electrical energy and the nervous tissue is not fully appreciated, it is known that application of an electrical field to spinal nervous tissue can mask certain types of pain transmitted from regions of the body associated with the stimulated nerve tissue. Applying electrical energy to the spinal cord associated with regions of the body afflicted with chronic pain can induce “paresthesia” (a subjective sensation of numbness or tingling) in the afflicted bodily regions. Paresthesia can effectively mask the transmission of non-acute pain sensations to the brain.
SCS systems generally include a pulse generator and one or more leads. A stimulation lead includes a lead body of insulative material that encloses wire conductors. The distal end of the stimulation lead includes multiple electrodes that are electrically coupled to the wire conductors. The proximal end of the lead body includes multiple terminals, which are also electrically coupled to the wire conductors, that are adapted to receive electrical pulses. The distal end of a respective stimulation lead is implanted within the epidural space to deliver the electrical pulses to the appropriate nerve tissue within the spinal cord that corresponds to the dermatome(s) in which the patient experiences chronic pain. The stimulation leads are then tunneled to another location within the patient's body to be electrically connected with a pulse generator or, alternatively, to an “extension.”
The pulse generator is typically implanted within a subcutaneous pocket created during the implantation procedure. In SCS, the subcutaneous pocket is typically disposed in a lower back region, although subclavicular implantations and lower abdominal implantations are commonly employed for other types of neuromodulation therapies.
The pulse generator is typically implemented using a metallic housing that encloses circuitry for generating the electrical pulses, control circuitry, communication circuitry, a rechargeable battery, etc. The pulse generating circuitry is coupled to one or more stimulation leads through electrical connections provided in a “header” of the pulse generator. Specifically, feedthrough wires typically exit the metallic housing and enter into a header structure of a moldable material. Within the header structure, the feedthrough wires are electrically coupled to annular electrical connectors. The header structure holds the annular connectors in a fixed arrangement that corresponds to the arrangement of terminals on a stimulation lead.
Spinal cord stimulation (SCS) is an effective therapy for chronic, intractable pain, and may be beneficial in treating other diseases including heart failure and vascular disease.
The SCS lead can shift, for example by up to 3 mm, relative to the spinal cord when a patient moves between postures, such as from a supine to prone position. Further, lead migration away from the implantation site can occur even in the absence of patient movement. When the lead shifts closer to or further from the spinal cord, an amount of stimulation energy that reaches the spinal cord increases or decreases, respectively, from a given initial stimulation amount of energy delivered from the lead. When the lead shifts closer to the spinal cord, the amount of stimulation energy that reaches the spinal cord increases. The increase may result in over-stimulation that may generate side effects that are uncomfortable for the patient, and may increase the likelihood of tissue damage due to high charge injection. Conversely, when the lead shifts away from the spinal cord, the amount of stimulation energy that reaches the spinal cord decreases. The decrease may result in under-stimulation that may cause a reduction or loss of paresthesia coverage and therapy for pain reduction. In addition to changes in lead-spinal cord distance, these shifts can produce changes in the electrical impedance of the surrounding tissue, which may shunt current towards or away from the targeted spinal cord region and change stimulation thresholds. Automatic adjustment of stimulation parameters, such as amplitude or pulse width, may compensate for these changes.
Evoked compound action potentials (ECAPs) are generated in the extracellular space by synchronous activation of a large number of nerve fibers in the dorsal column, and are sufficiently large to be recorded from electrodes in close proximity to the neurons on a SCS lead during stimulation. The ECAP signal may provide a suitable feedback signal for parameter adjustment across different lead positions, because the sensed ECAP signal amplitude is linearly related to the percentage of pain coverage with variation in stimulation current. It has been proposed to use the ECAP signal as a feedback signal in SCS.
However, the thus proposed method requires a preliminary training period to establish a “therapy map,” in which the ECAP signal amplitude is measured across a large number of stimulation amplitudes and patient postures. The training period is both time-consuming and incorrectly assumes a linear relationship in sensed ECAP signal amplitude between different lead-spinal cord distances.
A need remains for improved methods and systems for controlling spinal cord stimulation.